1. Field of Invention
Embodiments of the present invention relate to thermal energy harvesting (or thermal energy scavenging) techniques using ferromagnetic materials which provide a mechanical motion and piezoelectric, electroactive, electromagnetic, and/or magnetostrictive generator which converts the mechanical energy into electrical energy. In addition, shape memory alloys can be added to obtain enhanced operational effects.
2. Discussion of Related Art
The contents of all references, including articles, published patent applications and patents referred to herein are hereby incorporated by reference.
Thermal energy harvesting (or thermal energy scavenging) is defined as the conversion of heat energy, for example, but not limited to, using thermal gradation into usable electrical energy. Thermal gradations can be found where waste heat is involved such as oil pipe lines, engines, and electrical devices. The electrical energy harvested can then be used as a power source for a variety of low-power applications, such as, but not limited to, remote applications that may involve networked systems of wireless sensors and/or communication nodes, where other power sources such as batteries may be impractical [J. A. Paradiso, T. Starner, IEEE Pervasive Computing, Jan.-Mar.:18-27 (2005); S. Roundy, E. S. Leland, J. Baker, E. Carleton, E. Reilly, E. Lai, B. Otis, J. M. Rabacy, P. K. Wright, IEEE Pervasive Computing, Jan.-Mar.:28-35 (2005) ]. For these reasons, the amount of research devoted to power harvesting has been rapidly increasing [H. A. Sodano, D. J. Inman, G. Park, The Shook and Vibration Digest, Vol. 36: 197-205 (2004)].
Thermal energy has been successfully converted into electrical energy by using thermoelectric materials. These materials use the inverse Peltier effect, with which a voltage is generated when a temperature difference exists across a gap. The efficiency of thermoelectric devices is determined by the figure of merit (ZT) of the material, given by
      ZT    =                            α          2                          ρ          ⁢                                          ⁢          k                    ⁢      T        ,where α, ρ, k, and T are the Seebeck coefficient, electrical resistivity, thermal conductivity, and average operating temperature in absolute scale. The efficiency of the device is expressed as
      η    =                            Δ          ⁢                                          ⁢          T                          T          h                    ⁢                                                  1              +                              z                ⁢                                  T                  _                                                              -          1                                                    1              +                              z                ⁢                                  T                  _                                                              +                                    T              c                        /                          T              h                                            ,where the first term,
      Δ    ⁢                  ⁢    T        T    h  denotes Carnot efficiency and the second term,
                    1        +                  z          ⁢                      T            _                                -    1                      1        +                  z          ⁢                      T            _                                +                  T        c            /              T        h            denotes the limiting efficiency by thermoelectric materials. While useful, these materials produce little energy and efficiency for small temperature differences. [J. M. Gordon, Am. J. Phys., vol. 59, no. 6: 551-555 (1991)]. For example, even using the recently developed super lattice AgPbmSbTe2+m (ZT=2), the limiting efficiency becomes 18.3% for ΔT=100° C. and Tc=0° C., which is thought to be an optimistic view, given that commercially available chips (ZT<1) have no more than 11.4% efficiency [G. Yonghui, X. Jingying, Chem. J. on Internet, vol. 7, no. 2: 19 (2005)].
Recently, it has been proposed that thermal energy could be converted into electrical energy with high efficiency by coupling thermo-mechanical and electrical devices. [See U.S. provisional patent application No. 60/554,747 and published U.S. Application Number 2005/0205125, the entire contents of which are incorporated herein by reference It is claimed that improved conversion performance can be obtained by using a piezoelectric material and a thermostrictive material which exhibits a large thermally induced strain due to a phase transformation. One description of this relies on bending through a bimorph effect by placing it between a hot heat source and a cold heat sink.
Mechanical energy can be converted using a piezoelectric material, electroactive polymers, and electromagnetic induction. The amount of power accumulated via the piezoelectric harvester is proportional to the mechanical frequency which is exciting it [H. W. Kim, A. Batra, S. Priya, K. Uchino, D. Markley, R. E. Newnham, H. F. Hofmann, The Japan Society of Applied Physics, Vol. 43 9A:6178-6183 (2004)]. In addition, the voltage created in a piezoelectric material is proportional to the stress/strain. Therefore, increasing the operating frequency and applying higher stresses/strains are key to generating higher power. In addition to piezoelectric materials, there are other materials which convert mechanical energy into electrical energy such as electroactive polymer, electromagnetic induction, and magnetostrictive materials. Electroactive polymers such as PVDF convert mechanical energy into electrical energy using principles similar to piezoelectric materials. In electromagnetic induction, voltage is generated when there is a change in magnetic flux through any surface bounded by the path based on Faraday's law. Magnetostrictive materials change their magnetization depending on mechanical load. When combined with an electromagnetic coil mechanical energy can be converted into electrical energy. Therefore, there are many types of transmission of mechanical energy into electrical energy.
A shape-memory material is one that undergoes a change of crystal structure at a certain temperature, called the transformation temperature. Above this temperature the material has one crystal structure, called the Austenite phase, and below a relatively lower temperature it has another phase, called the Martensite phase. Each phase has different material properties such as Young's modulus and Yield strength. The low temperature structure of these types of materials allows the material to be easily deformed. Since a shape memory alloy has high energy density [P. Krulevitch, A. P. Lee, P. B. Ramsey, J. C. Trevino, J. Hamilton, M. A. Northrup, J. Microelectromech. Syst., vol. 4, no. 5: 270-282 (1996)], it can be added to an energy harvesting system to substantially enhance both the strain/stress and frequency developed in piezoelectric materials during thermal cycling. Ferromagnetic materials have a long term order of the magnetic moments resulting in formation of domains. Examples include transition metals iron, cobalt, nickel, and some of the rare earth metals such as gadolinium. Permanent magnetic moments in ferromagnetic materials result from atomic magnetic moments due to electron spin. An increase in temperature causes more atomic vibrations, which tend to randomize the magnetic moments. The saturation magnetization decreases gradually with increasing temperature, and drops to zero sharply at a temperature called the “Curie temperature.” Above the Curie temperature, Tc, ferromagnetic and ferrimagnetic materials become paramagnetic, at which only a negligible amount of magnetic moments exist.